The present invention relates to level sensing with a capacitive sensor.
Capacitive proximity switches are known. They can be used for level sensing.
As water approaches a metallic probe, the capacitance between the probe and the water changes. In particular if the probe is arranged parallel to the free surface of the water, as the water rises, there will be a steep rise in the capacitance as the free surface makes its final approach to the probe.
The capacitance will not increase on any further rise in the water above the probe. For detecting a further rise, duplication of the probe and associated detection circuitry is required.
The object of the present invention is to adapt capacitive level sensing to detect two levels in an improved manner.
According to one aspect of the invention there is provided a level sensor for sensing the level of a flowable material, the sensor comprising:
means for measuring capacitance;
a pair of electrodes, being:
a return electrode and
a measuring probe, arrangeable to exhibit a capacitancexe2x80x94measurable by the measuring meansxe2x80x94between itself and the return electrode, the capacitance varying in use with flowable material level,
wherein the measuring probe has:
two, electrically-common, level-detecting formations, each being arrangeable at least substantially parallel with a free surface of the flowable material, whereby two steep rises in the capacitance are measurable as the flowable material rises past first one of the formations and then the other formation and wherein the measuring means is adapted to indicate flowable material level in accordance with the presence/absence of the flowable material at the two probe formations.
Normally the xe2x80x9cflowable materialxe2x80x9d will liquid, but it should be noted that the level sensor may find application in sensing the level of other flowable materials, which are not liquids in the strict scientific sense as in the distinction between solids, liquids and gases. Sensors for such applications are included within the scope of the present invention.
Normally, the return electrode will be a discrete probe arrangeable in or adjacent the liquid. However, it can be envisaged that the return electrode is in effect earth, the measuring circuit then measuring capacitance to earth.
The two formations may be discrete probe elements which are electrically connected together. These may be configured as strips external to non-metallic vessels. For non-conductive liquids, internal probes, with coaxial outer return electrodes, can be utilized.
Alternatively the two formations may be formed or joined together on a single probe stem. Normally, in order to enable the measuring means to detect the liquid reaching the second formation with more certainty, the latter will have a larger capacitive surface area, typically at least three times that of the first formation. These formations may be of spirally formed wire, the spirals extending transversely of the stem.
For probes used internally of vessels, in particular for conductive liquids, the probes are preferably insulated.
Although the sensor will normally have only two formations, it is envisaged that it may have more formations, with the measuring means being adapted to indicate in accordance with a corresponding plurality of levels.
According to another aspect of the invention there is provided capacitance a measuring means for the level sensor of first aspect of the invention, the capacitance measuring means comprising:
means for measuring the capacitance between the pair of electrodes;
means for outputting a first signal if the capacitance is within a certain tolerance of a first threshold and for outputting a second signal if the capacitance is within a certain tolerance of a second threshold.
Normally the capacitance measuring means will be configured as a microchip.
To help understanding of the invention, a specific embodiment thereof will now be described by way of example with reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic perspective view of a level detector of the invention,
FIG. 2 is a plot of capacitance against liquid depth;
FIG. 3 is a similar view of an alternative probe for the level detector;
FIG. 4 is a side view of another pair of probes;
FIG. 5 is a block diagram of the counter of the level detector of FIG. 1
FIG. 6 shows in cross-section two further alternative level detector probes, in opposite halves of the drawing.